Police shot a man who threatened officers with a knife and he is in a critical condition in hospital, Victoria Police Superintendent David Clayton said.
Videos and photos shared on Twitter showed a burning vehicle and police officers in an altercation with the man clad in black.
More than 100 police have swarmed on the area.
Reports suggest the attacker was shot by police, and is now in a criticial condition.
No other suspects are being sought.
"We will keep an open mind as to whether there is any link, it's very early stages in the investigation", he said.
A total of three people were stabbed during the assault, police revealed during a press conference.
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Both Whitaker and Clovis are rooted in Iowa politics, and both ran for the 2014 Republican Senate nomination. But Senate Judiciary Chairman Chuck Grassley praised Whitaker. "I'm profoundly concerned about it", he said.
"Me and the managers ran out and that's when we saw the vehicle on fire and then we saw the guy on the floor and we wanted to help, there were two blokes helping him out already, he was face down pools of blood around his face", he told AAP.
It is not yet known if the incident is terror related.
The Friday incident took place blocks away from Flinders Street Railway Station, where a driver plowed into pedestrians previous year, injuring 18.
Members of the public are being urged to avoid the area with several streets reportedly placed on lock-down.
The incident occurred on Bourke Street just outside the Target Centre, near the intersection of Swanston St.
One of the injured people has a neck injury and is in a suspected critical condition, while another has a head injury, according to Ambulance Victoria.
Police are not looking for anyone further at this early stage.
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By late morning, the teen's Apple Watch and iPhone still showed her location on the dance floor, the uncle said. Ron Helus , a 29-year veteran of the Ventura County Sheriff's department who had confronted the gunman.
"It's so chaotic", witness Sarah told Tom Elliott.
'Police response was swift and overwhelming.
A man from Neapoli Cafe in Russell Place told the Herald Sun that he heard a lot of bangs and others reported hearing loud explosions.
"I really wanted to help but I was in shock, I didn't know what to do".
It was brought under control within about five minutes, a Metropolitan Fire Brigade spokeswoman said.
James Gargasoulas is accused of deliberately crashing into pedestrians, killing six and injuring 27 others.
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Bonucci floated a ball over the top of the defense and Ronaldo watched it over his shoulder before volleying into the top right corner.